Traditionally, the long-range transport of civilian and military cargo has been accomplished by either sea lift assets or large cargo-carrying aircraft. In the case of ocean-going vessels, large port facilities are required. Also, the time required to transport cargo by sea for long distances can be significant. In the case of large cargo-carrying aircraft, the size of the payload is limited. With this as a limitation, the costs to operate such aircraft (e.g. maintenance and fuel costs) can be prohibitive. The air transport of cargo, however, is relatively fast. Airframe designers, therefore, continue to look for ways to maximize the cargo carrying capability of aircraft, while maintaining or improving on the fuel efficiency and transport range of these aircraft. Typically, the engineering options that are considered include designing lighter aircraft, designing aircraft with more efficient engines, and designing aircraft with greater fuel carrying capability. Yet another option has been to develop aircraft that take advantage of certain natural phenomena associated with winged flight, specifically surface effects or “wing-in-ground” (WIG) effects.
To better understand the operational advantages and limitations of WIG vehicles or aircraft, it is important to first understand the underlying aerodynamics of the “wing in ground” effect. In general, when a fixed-wing aircraft flies near the earth's surface, an air cushion is created between the underside of the wing and the ground. In this flight environment, the air cushion imparts lift to the aircraft, while at the same time reducing drag on the aircraft. In actuality, the air cushion effect results from two physical phenomena often respectively referred to as “chord-dominated ground effect” and “span-dominated ground effect”. In particular, chord-dominated ground effect acts to increase the lift of the aircraft, while span-dominated ground effect acts to reduce the induced drag on the aircraft. The combined effect of the two phenomena is to increase the lift to drag, or L/D ratio, thereby allowing for more efficient flight on the “cushion of air”.
As can be appreciated by the skilled artisan, the span-dominated ground effect is most apparent in aircraft with a high aspect ratio wing. Specifically, the higher the aspect ratio, which is the wingspan divided by the average chord length of the wing, the lower the induced drag will be. Notably, as the wing gets closer to the earth's surface and the wing vortices are constrained and weakened at the wing tips, the “effective” aspect ratio of the wing increases beyond the geometric aspect ratio. As a result of the increase in this “effective” aspect ratio, the induced drag is reduced. Also, a reduction in drag is most pronounced when the ratio of the aircraft operational altitude to the length of the wingspan is on the order of 1:10. It can be mathematically shown that the net result of an increased “effective” aspect ratio, and a decreased aircraft altitude-to-wingspan ratio, can be a reduction in induced drag by as much as 50%.
Chord-dominated ground effect relies primarily on the fact that pressure under the wing increases as the aircraft flies nearer to the ground. Therefore, as the aircraft-to-ground distance decreases, the lift imparted to the aircraft from higher pressures under the wing significantly increases. Due to these combined effects (i.e. span dominated and chord dominated ground effects), WIG vehicles are able to transport heavier loads further, using less power and less fuel than would be possible for flight out of ground effect. Not surprisingly, WIG vehicles normally operate over water, where it is possible to fly close to the surface of the earth for extended distances without encountering obstructions.
A critical design concern for WIG vehicles is longitudinal stability and control as the aircraft transitions from WIG dominated flight to “free flight” at higher altitudes. In the transition between WIG flight and “free” flight, WIG vehicles have a tendency to dramatically “pitch up.” Traditional WIG designs have compensated for this “pitch moment” by employing various techniques for aerodynamic pitch control well known in the aircraft industry, to include: using very large vertical tail planes; optimizing the vehicle center of gravity; and modifying the wing design. Although many of these solutions are effective at controlling “pitch up,” many also increase the vehicle weight which adversely impacts both fuel efficiency and power.
In light of the above, it is an object of the present invention to provide an aerial vehicle that takes advantage of the “wing-in-ground effect” to optimize lift capability, vehicle speed, fuel efficiency and operating range. Another object of the present invention is to provide an aerial vehicle that integrates “lighter-than-air” lift and cycloidal propulsion subsystems into a WIG vehicle. Still another object of the present invention is to provide an aerial vehicle with improved longitudinal stability and control. Yet another object of the present invention is to provide an aerial vehicle that is simple to operate, relatively easy to manufacture, and comparatively cost effective.